Ureaplasma urealyticum is a member of the family Mycoplasmataceae and often observed in the vagina of pregnant women. In this study, we examined if ureaplasmal infection is associated with preterm delivery and whether ureaplasmal lipoprotein (LP) has the capacity to stimulate monocytes to produce pro-inflammatory cytokines relevant to preterm delivery in vitro. We selected 100 cases of normal delivery and 45 cases of preterm delivery at random. We generated a monoclonal antibody (mAb) against U.urealyticum urease that selectively and positively stained it in vaginal secretions of infected women but not of uninfected women. The preterm delivery group showed significantly higher incidence for vaginal infection with this bacteria than the normal delivery group. Since LP of Mycoplasma has a potent biological activity, we extracted ureaplasmal LP. THP-1 cells, human monocytic cells, produced interleukin (IL)-8, a potent pro-inflammatory cytokine associated with preterm delivery, and showed apoptotic cell death in response to the LP. These results suggested that U. urealyticum infection might play a causative role in preterm delivery via LP-induced of IL-8 production and apoptosis.